how to draw specified value contour

Hi,

I am new to NCL, I have tried to draw specified value contour, say, i want
to draw 5880gpm potential height in 500hPa plot.
I tried to set the values as follows, but it failed. Could you help and
tell me how to set?

----------------------------------------
  resH@cnMinLevelValF = 588.
  resH@cnMaxLevelValF = 588.
  resH@cnLevelSpacingF = 1.

Best wishes!

-- 
Tao Qing

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Received on Fri Oct 12 09:07:13 2012